1. Field of the Invention
This invention relates to drive systems on multi-engine marine vessels and, more particularly, to a drive system on a twin screw marine vessel that allows both engines or, alternatively, either engine to individually operate both propellers.
2. Discussion of the Related Art
Conventional motor driven boats often use two or more engines, wherein each engine is adapted to drive a single propeller. The use of twin screws, wherein one propeller is on the port side of the boat's centerline and the other propeller is on the starboard side of the boat's centerline, allows for easy maneuvering and turning of the boat at low speeds when the boats rudders have minimal affect. The use of twin screws is particularly useful when docking and maneuvering a boat in tight areas (e.g., a canal or marina).
While there are definite benefits to the use of twin screws in a marine vessel, the need to operate both engines, particularly at low speeds, is highly inefficient. Additionally, if one engine fails, the motor vessel can only operate on a single engine driving a single propeller. In this instance, it is difficult to steer, maneuver and dock the vessel. Moreover, inboard engines are typically mounted in side by side relation in a conventional motor driven boat in order to drive the corresponding port or starboard propeller. This arrangement with each engine offset relative to the centerline of the vessel is not to ideal for lateral stability.
My previous invention, as disclosed in U.S. Pat. No. 3,422,790 places both engines along the centerline of the boat to provide the best dynamic and static center of gravity. In at least one embodiment of my previous invention, two engines, arranged along the longitudinal centerline of a boat are connected to a drive system for driving two propellers. With the use of dog clutches, both engines or, alternatively, either engine, can operate both propellers. This allows for numerous advantages, particularly in the instances of failure to the engine.
Notwithstanding the numerous advantages of my previous invention, the ability to operate the boat on one engine driving twin screws at cruising speeds is problematic. It has since been discovered that a change in the gear reduction ratio when operating twin screws on the single engine allows the boat to obtain higher cruising speeds while operating on the single engine. Changing the gear reduction ratio between single engine operation and twin engine operation has been achieved with the use of under/overdrive units connected to the universal drive shafts for each propeller. Moreover, is has been discovered the use of two chain drives, one for each screw, is optimal in order to selectively operate both screws on twin engines or, alternatively, on either engine independently.